Add face removal to complete edge collapse (plus tests)

* add ability to remove faces (with test)
* add missing face removal to edge collapse (now handles adjacent triangular face which can't be merged)
* new edge collapse test for triangular faces
* in halfedge list, make add pair method internal (it doesn't maintain validity)
* add ability to remove pair of halfedges (not public either)
* add easy way of checking if edge is boundary